High-speed large-capacity radio communication is widely used for not only information and communication terminals such as mobile phones but also automobiles, etc. In high-speed large-capacity radio communication, high frequency signals are transmitted by an antenna transmitting and receiving information. As an antenna, for example, a wiring substrate comprising an electrical insulator layer and a conductor layer formed on the electrical insulator layer is used. In the wiring substrate, in many cases, conductor layers are formed on both surfaces of the electrical insulator layer, and the conductor layers are electrically connected by a plating layer formed on an inner wall surface of a hole (through-hole) penetrating through the electrical insulator layer. Further, the antenna transmitting and receiving radio waves is, as the frequency of the radio waves becomes high for example, formed on a wiring substrate called e.g. a printed circuit board having an electronic circuit formed thereon, utilizing the wiring pattern of the electronic circuit in many cases.
The wiring substrate used for transmitting high frequency signals is required to have excellent transmission characteristics, that is, small transmission delay and small transmission loss. In order to improve the transmission characteristics, it is necessary to use, as an insulating material forming the electrical insulator layer, a material having a low dielectric constant and a low dielectric dissipation factor. As an insulating material having a low dielectric constant and a low dielectric dissipation factor, a fluororesin such as polytetrafluoroethylene (PTFE) has been known (for example, Patent Document 1). However, a fluororesin such as PTFE has a low surface energy and is non-adhesive. Accordingly, if a fluororesin layer is used as the electrical insulator layer, particularly adhesion between an inner wall surface of a hole formed in the fluororesin layer and a plating layer can hardly be secured sufficiently. For example, partial peeling of the plating layer formed on the inner wall surface of the hole may cause conduction failure.
Accordingly, use of a fluororesin having acid anhydride residues has been proposed (Patent Document 2). Adhesion between the fluororesin layer and the plating layer is improved by the acid anhydride residues. Further, in a fluororesin layer, reinforcing fibers such as glass fibers are blended in many cases for the purpose of bringing its linear expansion coefficient to be closer to the linear expansion coefficient of the conductor layer thereby to suppress deformation such as warpage. However, if reinforcing fibers such as glass fibers are contained in the fluororesin layer, adhesion between the inner wall surface of the hole formed in the fluororesin layer and the plating layer tends to decrease.
As a pre-treatment when the plating layer is formed on the inner wall surface of the hole formed in the fluororesin layer, an etching treatment with an etching liquid having metal sodium dissolved in tetrahydrofuran has been known. By such an etching treatment, the fluororesin on the inner wall surface of the hole is partially dissolved to roughen the inner wall surface, whereby adhesion between the inner wall surface of the hole and the plating layer will increase by the anchor effect. Further, fluorine atoms on the inner wall surface of the hole are replaced by e.g. hydroxy groups to lower water repellency, and accordingly the plating layer tends to be formed on the entire inner wall surface of the hole. By such an etching treatment, adhesion between the inner wall surface of the hole and the plating layer will be sufficiently high even if the fluororesin layer contains reinforcing fibers. However, metal sodium used for the etching treatment may ignite (explode) by contact with water, and great caution is needed for its handling and storage area. Further, since an organic solvent is used in a large amount, there are problems of health damage of an operator by intake, post-treatment, etc.
As a method of forming a plating layer without conducting an etching treatment using metal sodium on the inner wall surface of a hole formed in a fluororesin layer containing glass fibers, a method of blending silica with the fluororesin layer has been proposed (Patent Document 3). However, if silica is blended, the dielectric constant of the fluororesin layer tends to be high, and electrical characteristics tend to be low.